Cryogenic grinding of copper

ABSTRACT

A process for abrasively grinding copper comprising (1) cooling the entire copper workpiece to a cryogenic temperature and (2) abrasively grinding the copper workpiece while it is at said cryogenic temperature. The workpiece is preferably immersed in liquid N2 during the abrasive grinding thereby preventing adhesive wear from occurring at relatively high metal removal rates.

Lightstone et a1.

1 1 CRYOGENIC GRINDING OF COPPER [75] Inventors: John BernardLightstone, White Plains, N.Y.; Richard Benedict Mazzarella,Indianapolis, Ind.

[73] Assignee: Union Carbide Corporation, New

York, NY.

[22] Filed: May 20, 1974 [21] App]. No.: 471,478

[52] US. Cl. 51/322; 83/15; 83/170 [51] Int. Cl B24b l/OO; B24b 55/02[58] Field of Search 51/2 F, 266, 267, 322, 51/284; 241/DIG. 13, DIG.22, 3, 15, l7, 18, 23; 82/1 C, 47; 83/15, 16, 170, 171

[56] References Cited UNITED STATES PATENTS 2,635,399 4/1953 West .151/322 2,917,160 12/1959 Turinsky 51/267 X 1.1.1 o n: E

2 CC I L|J I U) Aug. 26, 1975 2,924,873 2/1960 Knowles 51/267 X3,072,347 1/1963 Dombrowski 241/23 X 3,091,144 5/1963 Villalobos 83/153,430,390 3/1969 Wolcott 51/267 X 3,643.873 2/1972 George 241/33,750,272 8/1973 Gomond 51/284 UX 11/1973 Frable 241/23 X PrimaryExaminer-Donald G. Kelly Attorney, Agent, or FirmB. Lieberman 57ABSTRACT 3 Claims, 1 Drawing Figure FEED RATE (cm. secT'l PATENTEB AUBZ6 [975 A 7 .50 v NEE OH;

(1 X) HOHOzI HVBHS CRYOGENIC GRINDING OF COPPER BACKGROUND Thisinvention relates, in general, to abrasive processes, and moreparticularly, to an improved method for abrasively grinding coppermetal.

The abrasive grinding of metal is an operation in which hard, sharp andfriable abrasive particles are used as cutting tools. The abrasiveparticles are generally embedded in a wheel which is power driven as itcontacts the workpiece. In general, abrasive processes yield finesurface textures and precise workpiece di' mensions and are considered,for the most part, finishing operations, Among the abrasive processes incommon use are grinding, honing, lapping, superfinishing, abrasivemachining and abrasive cutting.

Grinding is the best known and most common abrasive process. Incontradistinction to most other metal cutting operations, grinding is aself-sharpening process. That is, as the abrasive particles wear duringcutting they either fracture or are torn from the bonding materialexposing new and sharp cutting edges. In order for the abrasiveparticles to maintain their sharp cutting edge, it is essential thatthey wear abrasively during normal operation. Abrasive wear occurs whena rough hard surface, such as an abrasive grit, contacts a softersurface (i.e. the workpiece) and cuts a series of grooves therein; theremoved workpiece material taking the form of long helicoidal chipswhich are generally thrown clear of the abrasion contact zone.

Adhesive wear is an undesirable type of wear which 7 may occur duringabrasive processing depending upon the rate of metal removal and thecomposition of the workpiece. It is basically a form of material removalwhich occurs when the fragments of the workpiece surface which areremoved by the abrasive particles adhere to such particles rather thanforming loose chips of metal. Microscopic observation reveals thatduring adhesive wear small elements of the workpiece come into contactwith the abrasive particles, adhere to said particles and when contactis broken, the break occurs not at the original interface but ratherwithin the individual elements of the workpiece. As a result; thegrinding surface becomes progressively loaded with the material beingabraded to the point where the abrasive particles are unable to cut theworkpiece efficiently. Consequently, adhesive wear results in a veryunsmooth workpiece surface and a marked increase in the shearing forcerequired for abrasion.

High purity copper is considered an extremely difficult material togrind because of its tendency to wear adhesively during abrasion. Thatis, when copper is abrasively machined it readily loads the grindingwheel, even at relatively low speeds corresponding to metal removalrates far lower than conventionally used for metals such as steel.Consequently, copper grinding is a relatively lengthy and expensivemachining operation. In an effort to prolong tool life and preventlocalized welding or adhesive wear during the machining of copper,coolants have been used to lower the temperature at the cut surface ofthe workpiece. Thus, coolants such as water and liquid CO have beendirected at the grinding wheel and the work surface in attempts toconduct heat away from the tool-'work interface. These techniques,however, have proven only partially successful because the resultingmetal removal rates are only slightly improved beyond those practical atroom temperature in the absence of a coolant spray. Consequently, copperabrading operations are presently incapable of being performed at metalremoval rates com-.

parable to those for iron and steel.

OBJECTS SUMMARY OF THE INVENTION These and other objects which willbecome apparent from the detailed disclosure and claims to follow areachieved by the present invention, one aspect of which comprises:

a process for abrasively grinding copper comprising the steps of: i

l. cooling the copper workpiece to be ground to a cryogenic temperaturesuch that substantially the entire workpiece is at said cryogenictemperature, and

2. abrasively grinding said copper workpiece while at said cryogenictemperature, thereby minimizing the tendency of the copper chips removedduring abrasion to weld to the grinding surface.

In a preferred embodiment of the present invention the copper workpieceis cooled by immersing substantially the entire workpiece in liquidnitrogen. The cooled workpiece is thereafter abraded while remainingimmersed in the cryogenic liquid.

The term abrasively grinding as used herein is intended to encompassabrasive processes such as surface grinding, honing, lapping,superfinishing, abrasive cutting and abrasive machining. Surfacegrinding, honing, lapping and superfinishing are finishing operationsintended to produce uniform high accuracy and fine finish on a surface.Abrasive machining is an abrasive process in which the primary aim ismetal removal while abrasive cutting is an abrasive process intended tosever metal parts.

The term copper as used herein refers to pure copper metal as well as tocopper alloys having either less than two weight percent of alloyingelements or copper alloys having a hardness below Rockwell B25irrespective of the percentage of alloying elements. Thus, the termcopper includes such alloys as the aluminum bronzes and berylliumcopper.

The term cryogenic temperature as used herein is intended to encompassthe range of temperatures corresponding to conventional cryogenic fluidssuch as liquid N and liquid CO Accordingly, cooling of the workpiece toa cryogenic temperature refers to temperatures below C with liquid Nbeing the preferred cryogen.

The invention is predicated on the discovery that the tendency of copperto wear adhesively during abrasive grinding can be substantially reducedor eliminated at relatively high rates of metal removal if the copperworkpiece is maintained at a sufficiently low temperature during thegrinding operation. Heretofore, it had been believed that cooling of theworkpiece was beneficial but only if it occurred at the grindinginterface, namely, within the zone of contact between the abrasiveparticles and the workpiece. It has now been discovered that bysufficiently reducing the temperature of substantially the entireworkpiece, a heat-sink effect is created which allows the frictionalheat generated at thework surface to be rapidly dissipated throughoutthe entire workpiece, thereby avoiding the transition from an abrasivewear process to an adhesive wear process at commercially practical metalremoval rates. Thus, the fact that a copper workpiece cooled with liquidnitrogen spray directed at the tool-work interface will experience anadhesion wear mechanism at relatively low metal removal rates, while asimilar workpiece immersed in liquid nitrogen will remain in theabrasive wear region at metal removal rates normally associated withiron or steel, is truly unexpected. More .over, the surprising nature ofthe result is underscored by the fact that similar type experimentsconducted with materials such as aluminum and iron showed no improvementwhatever when comparing the grinding of specimens immersed in liquidnitrogen with specimens which were cooled by directing a liquid coolantspray at the grinding wheel. Indeed, in tests conducted with iron,immersing the workpiece in liquid nitrogen proved to be a less preferredmode of cooling because grinding of the immersed workpiece wasaccompanied by a marked increase in the shearing force at the grindingsurface relative to that required when the grinding wheel was cooledwith a liquid spray.

DRAWINGS I The FIGURE is a plot of shear force vs. feed rate during theabrasive cutting of copper for three different modes of cooling theworkpiece.

DETAILED DESCRIPTION OF THE INVENTION The grinding operationscontemplated by the present invention include all of the types in commonuse such as surface, cylindrical, internal, centerless and off-handgrinding. The first four types of operations are used primarily toobtain accurate dimensions and good surface finishes. The material to beground is generally fed against the grinding wheel which is rotated at avelocity sufficient to attain a surface speed of from about 3000-5000ft/min. Finishing operations, such as honing, lapping and superfinishingare characterized by the extreme fineness of the abrasive particles.Off-hand grinding is used mainly where metal removal is of primeimportance and dimensions are not critical. In abrasive cutting, acut-off wheel is used to sever metal parts such as risers, sprues andflushing from castings. In abrasive machining, the primary aim is metalremoval, not surface finish.

Adhesive wear is a problem common to all abrasive grinding operations,but particularly, when the workpiece is a soft and ductile metal such ascopper. The presence of adhesive wear during copper grinding can bevisually detected by numerous methods. First, the abrasive surface canbe examined to determine the extent of copper loading thereon; second,the surface of the workpiece can be examined to determine whether theresulting finish is characteristic of abrasive or adhesive wear; andfinally, the chips of metal ejected from the tool-work contact area canbe examined to determine whether they are nodular or helicoidal inshape, the former being characteristic of adhesive wear and the latterindicating abrasive wear.

Cooling of the copper workpiece to cryogenic temperatures isaccomplished by immersing substantially ture; not merely the area ofcontact between the workpiece and the abrasive surface as inconventional practice. By cooling the entire workpiece in the mannerdisclosed, it is believed that a heat-sink effect is created whichprevents a rapid temperature rise at the worktool interface and theconcomittant loading of the abra sive wheel with copper chips.

EXAMPLE A series of experiments were conducted with annealed,electrolytic tough pitch copper to determine the effect of various modesof cooling the workpiece on the grinding operation. An abrasive cut-offmachine was modified to permit measurement of the shear force exerted bythe grinding wheel as copper barswere severed at varying rates of feed.A 10 inch diameter x 1/16 inch TC HRR wheel manufactured by theCarborundum Company was used in all the experiments. The wheel speed waskept constant at 2100 rpm.

Three sets of experiments were performed corresponding to three modes ofcooling the workpiece. In the first set of experiments, water wassprayed on to the cut-off wheel; in the second set, liquid nitrogen wassprayed on to the wheel; and in the third set the copper workpiece wasimmersed in a dewar flask containing liquid nitrogen such that the worksurface was about 1 inch below the level of liquid nitrogen. The resultsare shown in FIG. 1.

When cooling was accomplished by spraying water or liquid nitrogen on tothe wheel, the cutting mechanism is seen to be dependent on feed rate.At low feed rates, below. 0.24 cm secf, the shear force is described bythe equation:

where: S is the shear force in kg. wt. and f is the feed rate in cm.secf At higher feed rates, i.e. above 0.043 cm. secf, there is an abruptincrease in the shear force to a value approximately three times theshear force measured at the same feed rate when the copper specimen wasimmersed in a bath of liquid nitrogen. This discontinuity in the curveindicates a transition from an abrasive wear mechanism to an adhesivewear mechanism. The shear force of the specimen immersed in liquidnitrogen, on the other hand, was invariant with feed rate and isdescribed by equation (1) throughout the range of feed rates studied. Inother words, the workpiece immersed in liquid nitrogen remained in anabrasive wear region at feed rates up to three times that ordinarilyassociated with copper grinding in commercial practice.

What is claimed is:

1. A process for abrasively grinding copper comprising the steps of:

1. cooling the intended copper workpiece to a cryogenic temperature suchthat substantially the entire workpiece is at said cryogenictemperature, and

2. abrasively grinding said copper workpiece while at said cryogenictemperature thereby minimizing the tendency of the copper chips removedduring abrasion to weld to the grinding surface.

2. The process as in claim 1 wherein said workpiece is cooled byimmersing substantially the entire workpiece in a cryogenic fluid.

3. The process as in claim 1 wherein said cryogenic fluid is liquid N

1. A process for abrasively grinding copper comprising the steps of: 1.cooling the intended copper workpiece to a cryogenic temperature suchthat substantially the entire workpiece is at said cryogenictemperature, and
 2. abrasively grinding said copper workpiece while atsaid cryogenic temperature thereby minimizing the tendency of the copperchips removed during abrasion to weld to the grinding surface.
 2. Theprocess as in claim 1 wherein said workpiece is cooled by immersingsubstantially the entire workpiece in a cryogenic fluid.
 2. abrasivelygrinding said copper workpiece while at said cryogenic temperaturethereby minimizing the tendency of the copper chips removed duringabrasion to weld to the grinding surface.
 3. The process as in claim 1wherein said cryogenic fluid is liquid N2.